<!DOCTYPE html>
<style>
::-webkit-meter-bar { background: green }
</style>
<p>You should see a green, a default styled, and a blue meter below.</p>
<div><meter></meter></div>
<div id="host1"></div>
<div id="host2"></div>
<script>
host1.attachShadow({mode: 'open'}).innerHTML = "<meter></meter>";
host2.attachShadow({mode: 'open'}).innerHTML = "<style>::-webkit-meter-bar { background: blue }</style><meter></meter>";
</script>
